1. Field of the Invention
The present invention relates to a semiconductor laser device having a semiconductor laser component mounted thereon and a method of mounting the semiconductor laser component on a submount.
2. Description of the Related Art
When a semiconductor laser component is used in systems such as an optical communication system, an optical disk, a laser, a laser-beam printer and the like, such element is packaged suitably for its use. In packaging the semiconductor laser component, a direct bonding method of directly bonding the semiconductor laser component to a component, which is disposed in the package, such as a metal block, a circular stem, and the like can be used. However, since a structure obtained through this method is simple while the heat releasing property of the semiconductor laser component is not good, temperature thereof increases and thus a lifetime of the semiconductor laser component is shortened. For this reason, it is difficult to use the direct bonding method in a high-power semiconductor laser component.
Therefore, in order to solve the above problem, there has been a method for mounting a semiconductor laser component on a submount made of Si or SiC that is excellent in thermal conductivity and processability and also a method for bonding the resultant semiconductor laser device to a package. Recently, a bonding method using a submount having an excellent heat releasing property is widely used.
Now, a conventional method of mounting a semiconductor laser component will be described.
FIG. 1A is a view illustrating processes of mounting a semiconductor laser component, wherein reference numeral 1 indicates a semiconductor laser component, reference numeral 2 indicates a submount, reference numeral 3 indicates a bonding member made of eutectic crystal solder, reference numeral 4 indicates a collet, and reference numeral 5 indicates a heating table. First, as shown in FIG. 1A, the submount 2 is set on the heating table 5 and then the submount is heated up to a temperature of 183° C. or more at which the bonding member 3 on the submount 2 can be melted. In the meantime, the semiconductor laser component 1 is held through vacuum absorption by the collet 4, and is positioned on the mount surface of the submount 2.
Next, as shown in FIG. 1B, after the bonding member 3 is melted, the collet 4 holding the semiconductor laser component 1 descends and the semiconductor laser component 1 mounted on the bonding member 3 of the submount 2 is cooled. At that time, in order to secure sufficient bonding area between the semiconductor laser component 1 and the submount 2 sandwiching the bonding member 3 and to improve the heat conductivity by making the bonding member 3 thin, the semiconductor laser component is pressure bonded on the submount by the collet 4. Next, as shown in FIG. 1C, after the bonding member 3 is completely coagulated, the collet 4 releases the semiconductor laser component 1 and ascends.
The bonding method using the submount enables for the semiconductor laser component to be high-powered. However, the higher-powered semiconductor laser component results in enlargement of the submount, and widening of the bonding area between the semiconductor laser component 1 and the submount 2 sandwiching the bonding member 3.
In this regard, the enlargement of the submount 2 and the widening of the bonding area accompanied with the higher-powered semiconductor laser component have caused the following problems.
A volume of a substance is varied according to variation of temperature, and the rate of change (thermal expansion coefficient) of every substance is different. For this reason, when different substances (for example, semiconductor laser component and submount) are heated to bond to each other, since difference in temperature exists for a time period from the complete coagulation of the bonding member to the recovery to a normal temperature. Thus, a shearing force due to difference in thermal expansion coefficient is generated in the bonded portion and this shearing force causes a residual stress in the substances. Further, the residual stress is varied depending upon sizes and shapes of the substances and the residual stress generated in the semiconductor laser component 1 because of the following reasons increase with the enlargement of the submount 2.
FIGS. 2A to 2D are conceptual views illustrating variation in residual stress depending upon variation in size of the submount 2, in which FIGS. 2A and 2B are an overview and a conceptual view illustrating generation of stress when the submount 2 is small and FIGS. 2C and 2D are an overview and a conceptual view illustrating generation of stress when the submount 2 is large. When the thermal expansion coefficient of the semiconductor laser component 1 is larger than that of the submont, a force acts on the semiconductor laser component 1 in a direction of decreasing the bonding area and a force acts on the submount 2 in a direction of maintaining the bonding area.
When the submount 2 is small as in FIG. 2B, the force of maintaining the bonding area is a shearing force generated when the submount 2 below a bonding surface is compressed. When the submount 2 in FIG. 2D is larger than the submount 2 in FIG. 2B, the force of maintaining the bonding area is the shearing force generated when the submount 2 below the bonding surface is compressed and a shearing force generated when the remaining submount 2 at the periphery of the submount 2 in which the shearing force is generated is tensioned. When the semiconductor laser components 1 have the same size in the two cases, the shearing forces generated when the submount 2 is compressed are equal to each other. Therefore, the large submount 2 has the stronger force of maintaining the bonding area by the shearing force generated due to the tension, and this shearing force becomes stronger with increase in size of the submount 2. For this reason, the larger the submount 2 becomes, the stronger the shearing force acting on the semiconductor laser component 1 becomes. Therefore, the residual stress generated in the semiconductor laser component 1 increases with increase in size of the submount 2. Furthermore, the same is true of the semiconductor laser component 1 having a smaller thermal expansion coefficient.
Furthermore, if the bonding area between the semiconductor laser component 1 and the submount 2 sandwiching the bonding member 3 is made large in order to enhance the heat conductivity, the residual stress of the semiconductor laser component 1 increases for the following reasons. When the semiconductor laser component bonded to other substance is cooled, compression occurs around a center of the bonding surface. For this reason, the farther a place is from the center, the greater the difference in the amount of compression between different substances becomes and thus the shearing force becomes larger. If the bonding area increases, places away from the center are bonded, and thus the shearing force becomes larger than the area ratio. For this reason, the residual stress due to this shearing force increases.
As described above, in order to secure sufficient bonding area between the semiconductor laser component 1 and the submount 2 sandwiching the bonding member 3 and to improve the heat conductivity by making the bonding member 3 thin, the semiconductor laser component is pressure bonded on the submount by the collet 4. However, since the semiconductor laser component 1 and the submount 2 are bonded with the stress generated due to the pressure bonding, the stress due to the pressure bonding remains in the semiconductor laser component 1 even after the release of the pressure bonding by the collet 4. At that time, if the bonding area enlarges, the fluid resistance of the bonding member 3 increases and thus the force required for the pressure bonding increases. For this reason, the residual stress remaining in the semiconductor laser component 1 due to the pressure bonding increases with the enlargement of the bonding area.
In order to improve the heat releasing property, the submount 2 is bonded to the vicinity of a light emitting region of the semiconductor laser component 1. For this reason, the light emitting region is positioned at a position having a high residual stress in the semiconductor laser component 1.
In general, when current flows in the semiconductor laser component 1 by applying a stress of 100 MPa or more to the light emitting region, crystals are transposed, which deteriorates the laser characteristic or destroys the semiconductor laser component 1. This phenomenon occurs when the stress of 100 MPa or more is applied to a part of the light emitting region. In addition, the higher-powered semiconductor laser component 1 makes the residual stress in the light emitting region larger. For this reason, when current flows, the laser characteristic is deteriorated or the semiconductor laser component 1 is destroyed.
However, as described above, since the residual stress of the semiconductor laser component 1 is generated locally due to various causes and the distribution of stress varies due to sizes and shapes of the semiconductor laser component 1, the submount 2 and the collet 4, and the pressing force of the collet 4, etc. Accordingly, there is no correlation between the macroscopic deformation (bending) and the residual stress of the semiconductor laser component 1, which makes it difficult to specify the causes.